1. Field of the Invention
The invention relates to the operation of an electrostatic chuck for semiconductor wafer processing and, more particularly, to a method and apparatus for controlling chucking force in an electrostatic chuck using closed-loop control of the capacitance between a semiconductor wafer and an electrode or electrodes within the chuck.
2. Description of the Background Art
Semiconductor wafer processing equipment generally contains a vacuum chamber within which is mounted a wafer support pedestal or susceptor for supporting the wafer during processing. Electrostatic chucks are incorporated into the wafer support pedestal to facilitate retention of the wafer upon the support surface of the pedestal during processing. In general, an electrostatic chuck consists of a platen or a pedestal fabricated from a dielectric or semiconducting material. One or more conductive electrodes are embedded within this platen. The electrode or electrodes are spaced from, and parallel to, the wafer support surface of the pedestal.
In a monopolar electrostatic chuck, one electrode is embedded into the pedestal. To electrostatically retain the wafer, a DC or AC voltage is applied between the chuck electrode and the wafer. The applied voltage causes oppositely polarized charges to accumulate on the backside of the wafer and on the electrode (or on the chuck surface in a Johnsen-Rahbek type chuck). The opposite polarization of the charges produces an electrostatic force that retains the wafer upon the wafer support surface of the pedestal.
In a bipolar electrostatic chuck, two electrodes are embedded in the pedestal. A differential voltage is applied across the embedded electrode pair, while the wafer electrically floats. The differential voltage causes oppositely polarized charges to accumulate on the backside of the wafer and on each of the electrodes (or on the chuck surface in a Johnsen-Rahbek type chuck). The opposite polarization of the charges produces an electrostatic force that retains the wafer upon the wafer support surface of the pedestal.
In order to properly integrate the electrostatic chuck operation with the wafer process sequence, there is a need to monitor the wafer status with respect to the electrostatic chuck. For example, there is a need to monitor whether there is a wafer merely present on the chuck or whether the wafer is xe2x80x9cchuckedxe2x80x9d or electrostatically retained in the proper position. Such information is used by the semiconductor wafer processing equipment to control wafer processing and ensure that the pedestal or wafer is not damaged by initiation of a process while a wafer is not present or correctly chucked.
Various types of sensing methods have been developed for this purpose.
For example, U.S. Pat. No. 5,436,790 issued to Blake et al. on Jul. 25, 1995, discloses a wafer presence and clamp condition monitoring apparatus. In this apparatus, a circuit monitors the capacitance between two electrodes of a bipolar chuck embedded within a wafer support pedestal. The capacitance falls into one range with no wafer positioned upon the support surface and into a second range with a wafer in place but not clamped. Furthermore, the capacitance falls in a third range with the wafer held in place by an electrostatic chuck formed when the embedded pair of electrodes are energized with a differential DC voltage. The monitoring circuit senses when the capacitance of the system is in each of the ranges by converting the measured capacitance to a DC voltage that can easily be sensed and used to confirm wafer placement and clamping.
A commonly-assigned U.S. patent application Ser. No. 08/873,268, entitled xe2x80x9cMethod and Apparatus for Wafer Detectionxe2x80x9d, filed on Jun. 11, 1997 by Burkhart et al., discloses a method for detecting the presence and position of a semiconductor wafer upon a support pedestal, such as that for a ceramic electrostatic chuck electrostatic chuck. In particular, this application discloses electrodes being affixed to the surface of the wafer support pedestal, in addition to those embedded in the electrostatic chuck. The wafer""s presence, position, and chucking condition can be determined through capacitance measurements performed between an embedded electrode or electrodes of the electrostatic chuck and other surface electrodes. This commonly-assigned reference is hereby incorporated by reference.
An electrostatic chuck controller typically employs an open loop control system, in which the chucking operation is controlled by applying a constant voltage to the chuck electrode(s). However, such a control scheme does not take into account any chuck to chuck variations arising from the chuck manufacturing process, time degradation in the electrostatic chuck itself, or other external disturbances during operation of the electrostatic chuck. These variations, if not accommodated, will in turn lead to undesirable variability in the electrostatic chuck performance.
Therefore, a need exists in the art for an improved control of electrostatic chuck that can dynamically compensate for changing or variable chucking characteristic in order to achieve optimal electrostatic chuck performance.
The disadvantages associated with the prior art are overcome by the present invention of a dynamic feedback circuit that measures the capacitance between a wafer and the electrode(s) of an electrostatic chuck, then adjusts the chucking voltage to maintain a constant capacitance measurement. By dynamically adjusting the chucking voltage to maintain a constant capacitance, the invention maintains a constant chucking force on the wafer. A constant chucking force is desirable for many reasons. For one, a constant chucking force corresponds to a constant sealing force about the periphery of the wafer. A constant sealing force creates a constant backside gas leak rate which results in a constant pressure in the gap region between the wafer and the ESC. This state is desirable since it results in a constant heat transfer between the wafer and the electrostatic chuck.
More specifically, an illustrative embodiment of the invention contains a monopolar electrostatic chuck having a pair of coplanar electrodes embedded in a dielectric material. To chuck a wafer, the electrode pair is coupled to the same voltage from a DC power supply, while the wafer is coupled directly to a second potential (e.g., ground). The potential difference between the electrodes and the wafer creates an electrostatic force that retains the wafer upon the chuck surface. A capacitance measuring circuit measures the capacitance between the electrode pair. The measured capacitance has a nominal value without a wafer present and the capacitance value changes when a wafer is positioned upon the chuck, when a wafer is chucked and, most importantly, the capacitance changes in direct response to the chucking force applied to the wafer. Furthermore, the capacitance has an anomalous value when a wafer is improperly chucked, e.g., a wafer is offset from the center of the chuck. The invention monitors the capacitance value to derive an optimal chucking voltage that maintains an optimal chucking force upon the wafer for a given application.